1. Field of the Invention
The present invention relates to a method for rendering an inorganic fine oxide powder hydrophobic and more particularly to a method for treating the powder which has a hydrophilic property by allowing an organo-polysiloxane to react with the surface of the powder to change it to be hydrophobic.
2. Description of the Prior Art
Fine oxide powders of silica, alumina, titanium oxide, etc. (hereinafter will be called the powder) are useful industrial materials which are widely in use for pigments, fillers, thickeners, etc. As well known, the surface of the powder is covered with a hydroxyl group and is oleophobic showing a strong affinity for water. The powder thus has a low degree of affinity for organic compounds and, accordingly, the low affinity and a poor dispersibility of the powder often causes inconveniences when it is used with an organic dispersion medium, a high polymer compound or the like.
To eliminate this shortcoming of these powders, there have been reported many treating methods for rendering their surfaces hydrophobic. In respect to the effect attainable by such methods reported, i.e. the degree of the hydrophobic property imparted to the powders and their durability in terms of heat resistance and water resistance shown in the use of them and with safety for the human also taken into consideration, a method of rendering the powder hydrophobic by using an organic silicon compound as hydrophobic property imparting agent and by allowing it to react with the surface hydroxyl group of the powder saliently excels others. Typical known examples of such a method includes the manufacture of a hydrophobic silica product carried out by treating a fine powder of silica.
In the prior arts, an organic silicon compound having a functional group of a very high reactivity with the surface hydroxyl group of the powder, such as an organo-chlorosilane or a silazane is brought into mixing contact with the powder; or with acid such as hydrogen chloride, hydrogen fluoride, fatty acid fluoride etc. or alkali such as ammonia, amine, etc. employed as catalyst, an organo-polysiloxane which is a polymer having the siloxane bond of an organic silicon compound or preferably a cyclic organo-polysiloxane such as hexamethyl cyclo-trisiloxane, octamethyl cyclotetrasiloxane or the like is allowed to react with the surface hydroxyl group of the powder. However these methods of the prior arts have been presenting many problems such as: the degree of the hydrophobic property imparted to a powder, and the adsorbed residue of an organic or inorganic acid, as a catalyst or which is produced as by-product through the reaction of the treatment, which remains in the treated powder; and the problems in connection with handling and the cost of the organo-silicon compound employed as hydrophobic property imparting agent.
For example, a Japanese patent publication No. 45-12567 discloses a method wherein the powder which has an adsorption water content is treated by carrying out a reaction with a cyclo-trisiloxane used as hydrophobic property imparting agent and with ammonia or an amine added as catalyst at a temperature between 15.degree. and 170.degree. C. In accordance with this method, since the cyclo-trisiloxane and the catalyst used are compounds of a low boiling point, the reaction is carried out in a closed system and the temperature must be raised in order to increase the reaction velocity and to obtain a higher hydrophobic property. Therefore, the use of a pressure reactor is required in practicing this method. Further, in this method, the powder and the components required for imparting a hydrophobic property must be mixed more homogeneously. The patent thus also discloses a method for effecting such homogeneous mixing which calls for the use of an organic solvent such as toluene or a method of contacting the powder with the vapor of cyclo-trisiloxane and ammonia or the amine. However, the use of an organic solvent necessitates a troublesome process for removal and recovery of the solvent from the powder after treatment. Besides, it is relatively difficult, even under a high vacuum and at a high temperature, to completely remove a slight amount of the solvent remaining in the treated powder.
Further, in order to quickly contact with the powder and vapor of the components for imparting a hydrophobic property to the powder, heating is necessary and the process must be carried out under pressure, because it takes an excessively long period of time for actual application of the method to leave them at room temperature. Then the use of an organo-polysiloxane and an amine that have a high boiling point becomes impossible.
The specification of a U.S. Pat. No. 3,649,588 discloses a method wherein, for securing a necessary quantity of a catalyst amino compound, particularly that of ammonia, the powder is treated with an organo-polysiloxane under the pressure of the catalytic amino compound. According to the method of using under high pressure, the treatment is carried out immediately with heating to a temperature above 60.degree. C., and of using under low pressure, the treatment is carried out over a long time at a temperature between 20.degree. and 60.degree. C. and then with heating above 100.degree. C. In accordance with the patent, however, the adsorption water content of the powder to be treated is not desirous in general, with the exception of some specific cases, and the allowable quantity of the adsorption water content of the powder is limited to an extremely small quantity ranging from 0.1 to 1.0% by weight even in such specific cases.
As mentioned in the foregoing, the conventionally known arts have been presenting various problems. In addition to such problems, the long treatment reaction time and the necessity of an operation under pressure which calls for complex facilities also present additional problems for industrialization. Further, since the bulk density of the powder to be treated is relatively small in general, such an operation under pressure necessitates the use of a very large reactor for the treatment. Particularly, for carrying out the treatment on an industrial scale, the use of a pressure vessel is very disadvantageous in terms of economy. Further, in cases where ammonia or the like is heated under pressure, such a process presents a greater hazard than a process under normal pressure. Thus the prior arts for imparting a hydrophobic property to the powder necessitate an operation under pressure and have been very unsatisfactory in terms of industrial applications and economy.